Educational  ·  Mechanism-Led

MK-677 - what the science actually shows

MK-677 (Ibutamoren) is an oral growth hormone secretagogue studied for growth hormone and IGF-1 elevation. Not FDA-approved. Evidence base: multiple human studies on growth hormone and IGF-1 levels and body composition.

An orally active ghrelin receptor agonist that elevates GH and IGF-1 without injection. One of the most clinically studied GH secretagogues, with multiple Phase II and Phase III trials. Not a peptide itself, but grouped with GH secretagogues. Here is the evidence - in plain English.

Oral
Route of administration
~24 hr
Half-life (pharmacokinetic)
Not
FDA approved
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On this page
  1. What MK-677 is and where it comes from
  2. Mechanism, in plain English
  3. What the clinical evidence shows
  4. FDA status
  5. Common synonyms
  6. Safety and adverse-event landscape
  7. Frequently asked questions
Mechanism
Non-peptide GHSR-1a agonist (ghrelin mimetic); orally active; amplifies pulsatile GH secretion and sustains IGF-1 elevation.
Evidence Level
Multiple Phase II and Phase III human clinical trials published. No FDA approval. Among the best-studied GH secretagogues.
FDA Status
Not approved for any human indication. Phase III trials for hip fracture recovery (NCT00491) and GH deficiency completed; no NDA submitted.
Landmark Papers
Nass 2008, NEJM; Chapman 1996, NEJM
Class / Type
Non-peptide spiroindane ghrelin mimetic; orally bioavailable GHSR-1a agonist; growth hormone secretagogue
Last Reviewed
August 9, 2026

What MK-677 is - and where it comes from

MK-677 (chemical name: ibutamoren mesylate) is a small molecule developed by Merck in the 1990s. Despite being grouped with growth hormone secretagogues and often called a "peptide" in community discussions, it is technically a non-peptide spiroindane compound. It was designed to address a key problem in GH therapy: the need for injection. MK-677 is orally bioavailable, which makes it uniquely convenient compared to injectable GHRPs and GHRH analogs.

MK-677 works by mimicking ghrelin's action at the GHSR-1a receptor - the same receptor that Ipamorelin targets - but through a structurally distinct small-molecule scaffold rather than a peptide sequence. Its oral availability enabled longer-duration clinical trials that generated a substantial human evidence base.

MK-677 has more published human clinical trial data than almost any other compound in the GH secretagogue category. The two pivotal NEJM publications represent a rare standard of evidence for this class.

Peptexa editorial note

Mechanism, in plain English

Ghrelin is the “hunger hormone” produced primarily in the stomach. One of its roles is to signal the pituitary to release growth hormone. MK-677 mimics ghrelin at the GHSR-1a receptor, triggering GH pulses without requiring ghrelin itself to be elevated. Because of its ~24-hour half-life, MK-677 produces sustained GH and IGF-1 elevation rather than a single brief pulse.

GHSR-1a
Ghrelin receptor agonism - pituitary GH secretion trigger
IGF-1↑
Sustained liver IGF-1 production - measurable in 24-hr blood samples
Oral
Non-peptide scaffold - survives GI degradation; no injection required
Appetite↑
Ghrelin mimicry also activates appetite pathways - increased hunger is common

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What the clinical evidence shows

MK-677 has one of the strongest published human evidence bases of any compound in the GH secretagogue category. Two landmark publications in the New England Journal of Medicine anchor the literature:

Phase III trials for hip fracture recovery in older adults (evaluating muscle preservation and functional outcomes) were completed but MK-677 was not advanced to an NDA by Merck. The program was discontinued for commercial rather than safety reasons, according to available disclosures.

The two-year Nass (2008) NEJM study is genuinely rare in this field - most GH secretagogues have only short-term or preclinical data. The insulin resistance finding is the most clinically significant adverse signal from that body of work.

Peptexa editorial review

FDA status

MK-677 is not FDA-approved for any human indication. Merck did not file a New Drug Application despite completing Phase III trials. MK-677 is not classified as a peptide compound but was included in broader regulatory discussions around GH secretagogues in 2026 advisory panel sessions. Last reviewed 2026-08-09.

Common synonyms researchers use

MK-677  ·  Ibutamoren  ·  Ibutamoren mesylate  ·  L-163,191  ·  MK-0677  ·  Nutrobal (informal community name)

Safety & adverse-event landscape

In published clinical trials, the most consistent adverse events were increased appetite (due to ghrelin pathway activation), peripheral edema (water retention), fatigue, and increases in fasting blood glucose. The Nass (2008) two-year study specifically documented a statistically significant increase in insulin resistance in older adult subjects. This is the most clinically important safety signal in the published literature and warrants discussion with a healthcare provider for any individual with metabolic considerations.

MK-677 also elevates prolactin modestly in some studies, and the sustained GH/IGF-1 elevation raises the same theoretical concern about undetected neoplasm acceleration that applies across all GH secretagogues. Peptexa does not provide dosing guidance. Consult a qualified healthcare provider.

Frequently asked questions

No. MK-677 is a non-peptide small molecule - a spiroindane compound. It mimics ghrelin's action at the GHSR-1a receptor but is not a peptide chain. It is grouped with GH secretagogues due to its mechanism, not its chemical structure.
No. Despite multiple Phase II and Phase III trials, Merck did not file a New Drug Application for MK-677. It is not approved for any indication.
The most significant documented adverse effects from the Nass (2008) two-year trial were increased fasting blood glucose and insulin resistance, peripheral edema, and increased appetite. These were more pronounced in older adults than in younger study populations.
MK-677's primary practical advantage is oral bioavailability. Pharmacokinetically, it produces a longer-duration, flatter GH elevation curve compared to short-acting injectable GHRPs that produce sharper, briefer pulses. Whether sustained vs. pulsatile GH patterns have meaningfully different physiological effects in humans has not been definitively studied head-to-head.
No. Peptexa is an educational resource only. We do not sell any compounds.

Related on Peptexa

Ipamorelin - injectable GHSR-1a agonist Sermorelin - FDA-approved GHRH analog Take the peptide education quiz Peptexa home - all peptide references

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